The invention concerns a method for the open-loop and closed-loop control of an internal combustion engine, in which, during normal operation, the rail pressure is automatically controlled in a closed-loop rail pressure control system by a suction throttle on the low-pressure side as a first pressure regulator, and, at the same time, the rail pressure is acted upon with a rail pressure disturbance variable by means of a pressure control valve on the high-pressure side as a second pressure regulator by virtue of the fact that a pressure control valve volume flow is redirected from the rail into a fuel tank by the pressure control valve on the high-pressure side.
In an internal combustion engine with a common rail system, the quality of combustion is critically determined by the pressure level in the rail. Therefore, in order to stay within legally prescribed emission limits, the rail pressure is automatically controlled. A closed-loop rail pressure control system typically comprises a comparison point for determining a control deviation, a pressure controller for computing a control signal, the controlled system, and a software filter in the feedback path for computing the actual rail pressure from the raw values of the rail pressure. The control deviation is computed as the difference between a set rail pressure and the actual rail pressure. The controlled system comprises the pressure regulator, the rail, and the injectors for injecting the fuel into the combustion chambers of the internal combustion engine. For example, DE 103 30 466 B3 describes a common rail system of this type, in which the pressure controller acts on a suction throttle by means of a control signal. The suction throttle in turn sets the admission cross section to the high-pressure pump and thus the volume of fuel delivered.
The unprepublished application DE 10 2009 031 527.6 also describes a common rail system with automatic control of the rail pressure by means of a suction throttle on the low-pressure side as a first pressure regulator. This automatic pressure control in the common rail system is supplemented by a pressure control valve on the high-pressure side as a second pressure regulator, by which a pressure control valve volume flow is redirected from the rail into the fuel tank. A constant leakage of, for example, 2 liters/minute is reproduced in the low-load range by means of activation of the pressure control valve. Under normal operating conditions, on the other hand, no fuel is redirected from the rail. The pressure control valve volume flow is determined on the basis of a set volume flow with a static and a dynamic component. In the computation of the dynamic component and the computation of the control signal for the closed-loop rail pressure control system, the actual rail pressure is a critical input variable. Therefore, a defective rail pressure sensor or an error in the signal acquisition of the rail pressure results in a false actual rail pressure and causes faulty activation of both the suction throttle as the first pressure regulator and the pressure control valve as the second pressure regulator. The cited document fails to provide any fault safeguard in the event of failure of the rail pressure sensor.